Gesture-based user interaction allows a user to control an electronic device by making gestures such as writing letters to spell words, swatting a hand to navigate a selector, or directing a remote controller to direct a character in a video game. One way to provide for such interaction is to use a device such as a mobile phone or tablet computing device equipped with a touch screen for two-dimensional (2-D) touch input on the touch screen. But, this can have the disadvantage that the screen is typically occluded while it is being touched, and such devices that include touch screens are also comparatively expensive and somewhat large in their form factors. Another way is to use depth cameras to track a user's movements and enable three-dimensional (3-D) gesture input to a system having an associated display, and such functionality has been provided in certain smart televisions and game consoles. One drawback with such three-dimensional gesture tracking devices is that they have high power requirements which presents challenges for implementation in portable computing devices, and another drawback is that they typically require a fixed camera to observe the scene, also a challenge to portability. For these reasons, there are challenges to adopting touch screens and 3-D gesture tracking technologies as input devices for computing devices with ultra-portable form factors, including wearable computing devices.